It is of primary importance to the operation of an elevator system that the elevator system should function faultlessly and above all in a predictable manner. Especially in situations where various systems, e.g. remote monitoring systems, are connected to the safety circuits of already existing elevators, it is necessary to make sure that the operation of the safety circuits of the elevator system will meet the required safety standard even after the connection of said systems.
Elevator systems employ various monitoring devices and methods to ensure the safety of the elevator. One of these is the so-called electric safety circuit. The safety circuit consists of safety device contactors connected in series. If any one of the safety devices breaks the safety circuit, then the elevator will stop or will not start moving. The safety circuit monitors e.g. the car doors, hoistway doors, locks, etc. If e.g. the elevator car doors are open, then the safety circuit is open.
FIG. 1a presents an example of the structure of a safety circuit. In FIG. 1a, there are three safety contactors 10, 12, 14 connected in series. The safety circuit is connected to main contactors 16 and a monitoring card 106, which is presented as a highly simplified resistance circuit. The main contactors 16 and the monitoring card 106 are connected to common ground 18 (neutral ground). The worst fault situation that may be caused by the monitoring card is illustrated in FIG. 1b, where the neutral conductor connected to neutral ground 18 is broken (110). In this situation it is possible that the fault causes a current to flow through the main contactors that will be sufficient to keep the main contactors energized “while the safety circuit is open”.
Let us assume that each one of the resistances 100, 102, 104 has a magnitude of 300 k•. In this case, the smallest over-bridging resistance has a magnitude of 450 k•. In other words, two parallel 300k• resistances are in series with a third 300 k• resistance. FIG. 1c presents a circuit corresponding to the circuit in FIG. 1b. The resistance 112 has a magnitude of 450 k•. If the safety circuit voltage (Umax) is 230Vac, then the largest possible fault current will be about 0.5 A.
On the basis of the above-mentioned factors, it is possible that the fault current produced in a fault situation will be sufficient to keep the main contactors energized. If the main contactors remain energized even if the safety circuit is open, then the elevator does not meet the safety regulations.
Elevator safety regulations recommend that the neutral ground of the safety circuit should be connected via an analyzing card, such as e.g. a remote monitoring card, to neutral ground. The recommendations given by elevator regulations define the safest way of implementing the connection of an analyzing card to the safety circuits. If a deviation from this is opted for, then a corresponding safety level has to be proved via a risk analysis. The normal recommendation for a circuit to avoid a bypass current situation is to take the return current from the main contactors via the neutral conductor of the analyzing card to neutral ground, thus making it impossible for the safety circuit to be incorrectly bypassed in a fault situation. Such a circuit is presented in FIG. 1d. However, in many existing elevator control systems it is often difficult to change the connections afterwards.